The production and utilization of bioenergy using biomass are becoming increasingly important to cope with petroleum resource depletion and emissions of global warming gases arising from the use of petroleum resources.
Bioenergy as substitute energy for petroleum can be produced by extracting oil components from hydrocarbon-rich plants and seaweeds. As an alternative, bioenergy can be produced by chemically or biologically treating biomass to separate sugar components, which are then converted to the corresponding alcohols through fermentation. Lignocellulosic biomass is a non-edible part of plants and is composed of lignin, cellulose, and hemicellulose. The separated cellulose and hemicellulose can be converted to monosaccharides, including pentose (e.g., arabinose and xylose) and hexose sugars (e.g., glucose), which are readily fermented to the corresponding alcohols by microorganisms. Accordingly, a typical process for producing bioalcohol from biomass consists of pretreatment, hydrolysis (or saccharification), fermentation, and distillation steps. According to the pretreatment step, biomass is finely divided, the biomass tissues containing cellulose, hemicellulose, and lignin, which are tightly bound to one another, are loosened, and the hydrolyzable cellulose and hemicellulose are separated from the biomass tissues at a molecular level. In the hydrolysis step, the cellulose and hemicellulose are converted to sugars. The sugars are converted to the corresponding alcohols in the subsequent fermentation step, and the alcohols are isolated and concentrated in the final distillation step.
Many methods have been proposed for the pretreatment of biomass, for example, by steam explosion, the use of dilute acids, such as dilute sulfuric acid or hydrochloric acid, the use of concentrated acids, such as concentrated sulfuric acid, the use of alkalis, such as sodium hydroxide or ammonia, and the extraction with organic solvents. The steam explosion does not use any harmful chemicals but is limited in extracting cellulose and hemicellulose with steam. Another disadvantage of the steam explosion is that harmful components are produced from the raw material for a prolonged pretreatment time. The use of dilute acids is advantageous in that even hemicellulose can be extracted but requires high pretreatment temperature and pressure conditions and a long operation time, inevitably resulting in the production of considerable amounts of toxic substances, such as furfurals and phenols. The use of concentrated acids enables the extraction of cellulose and hemicellulose within a short time and can thus minimize the production of toxic substances during pretreatment. However, this method incurs a considerable cost in recovering the acids after use and regenerating the acids to high concentrations. By the use of alkalis, lignin is dissolved and cellulose and hemicellulose are caked into a solid mass, which is hydrolyzed in the subsequent step. According to this method, a large amount of liquid alkali waste is generated, lignin is difficult to completely remove from the solid cake as a raw material for hydrolysis, and many problems are encountered in handling the solid cake during processing in an apparatus. The use of organic solvents causes less loss of cellulose and hemicellulose during extraction but incurs a high recovery cost. The organic solvents whose boiling points are low tend to be lost.
Particularly, the use of concentrated acids is advantageous in pretreating various biomasses due to the outstanding ability of concentrated acids to extract polysaccharide structures like cellulose and hemicellulose within a short time and convert the polysaccharide structures to molecular units. In addition, concentrated aqueous acid solutions can also be used for the subsequent saccharification (or hydrolysis) of extracted polysaccharides to make hydrolyzates after pretreatment, achieving the highest hydrolysis yield among methods for producing biomass hydrolyzates. Successful separation of the aqueous acid solutions from the hydrolyzates after use in the pretreatment and hydrolysis steps and sufficient concentration of the aqueous acid solutions enable recycling of the acids. Therefore, the use of concentrated acids would be very useful for biomass pretreatment and subsequent hydrolysis.
A continuous adsorption-separation method based on the use of simulated moving bed (SMB) columns is known in which an acid hydrolyzate (acid-containing hydrolyzate) obtained by pretreatment and hydrolysis with a concentrated aqueous acid solution (concentrated sulfuric acid) are separated into sugars and the acid, which are then recovered. This method uses a system having a construction in which a plurality of chromatography columns for selective adsorption and separation of sugars or sulfuric acid are installed and valves and a pump for continuous sample supply and continuous product discharge are connected to each column. Due to this construction, the system enables continuous separation of sugars and sulfuric acid from the hydrolyzate. However, the separation of sugars and sulfuric acid requires the use of a large amount of water as a carrier, causing dilution of sulfuric acid and sugars to several wt % (hereinafter, the concentrations of sulfuric acid and sugars are expressed as weight percentages (wt %)). For sulfuric acid recycling, the dilute aqueous solution of several % sulfuric acid should be concentrated by distillation. For efficient sugar fermentation, the sugar concentration of the discharged hydrolyzate should also be increased to at least about 10%. However, this concentration involves a high energy cost and is thus inefficient. The SMB system uses chromatography columns packed with expensive adsorbents and its construction is complicated, making the operation very complex. When the acid hydrolyzate has a high acid concentration, the adsorbents undergo severe swelling or shrinkage upon acid adsorption, resulting in low separation efficiency. The repeated adsorption-desorption cycles deteriorate the durability of the adsorbents. For these reasons, the SMB system is not easy to apply to pretreatment and hydrolysis processes using concentrated sulfuric acid.
Thus, there is a need for an economically efficient method by which an acid and sugars are separated from an acid hydrolyzate while minimizing loss of the sugars from the acid hydrolyzate, and at the same time, a concentrated aqueous sulfuric acid solution can be recovered for reuse.